Hey,
Only just seen this thread but from what I have read my opinion is you shouldnt of even started the car if its been standing about for 1 year as it will be full of water and everything will be damp (thats if it was stored outside)
You should of got some help and had the oil fully changed with a new oil filter and a new fuel filter to start and fuel tank cleaner from Halfords and then once that was all done then fired the car up!
If the car had cut out you should of then looked at the obvious which has been posted many times, replace the spark plugs, doesnt really matter which as long as they are for the Cinquecento such as Champion EON2's or some even more basic ones, also change the HT leads.
If the car again cuts out then its obviously abit more serious for example the sensors as mentioned might be dodgy so and for it to cut out it could be the Lambda sencor which is always known to go. If its not Sensors then maybe the Throttle body is not letting enough fuel in which could be a blocked fuel pipe or even a leak in the pipes "can you smell fuel?"
Whatever the problem is I advise you to get help from a garage as it seems you are doing more damage than is needed, i.e. destorying the sump bleeding nut!
It deosnt sound major whats wrong because it still starts, I really would advise you to have a garage to have a peak, even Fiats!
Good luck
Regards
Adam